1. Field of the Invention
The present invention relates to a chiplessly thread-forming screw used, in particular, for screwing metal sheets and having a stem carrying at least regionwise, a thread, a tip provided at one end of the stem, and a head provided at an opposite end of the stem, with the stem having a pointed conical section extending from the tip in a direction of the head and a cylindrical section arranged between the pointed section and the head.
2. Description of the Prior Art
U.S. Pat. No. 4,844,676 discloses a self-penetrating screw having a head and a stem which adjoins the head and has, at its end remote from the head, a tip. The stem has a cambered section that tapers toward the tip, and a cylindrical section that adjoins the cambered section in the direction of the head. The cambered section has a first thread with thread turns which are spaced relatively far form each other, whereas the cylindrical section has a second thread with thread turns arranged relatively close to each other. A cambered transitional region is provided between the first, cambered section and the second, cylindrical surface.
The drawback of the above-described self-penetrating screw consists in that it can be used only for very thin metal sheets such as, e.g., used in an automobile industry. This screw is not suitable for screwing together a stack of several, arranged one above another, metal sheets.
German Patent DE 196 37 969 discloses a chiplessly hole- and thread-forming screw having a head with a tool receptacle and an adjoining the tool head, thread stem that passes in a conical tapering section. In this screw, a first region of the conical section having a polygonal cross-section adjoins a substantially cylindrical thread stem, whereas the second region forms a tip cone, with the cone angle of both the first and second regions amounting to about 60°.
The drawback of the screw described immediately above consists in that it likewise can be used for very thin sheets having a thickness of about 1.5 mm or lower. Further, the use of this screw requires application of high press-on forces, which are provided in the automobile industry by setting robots. For screwing a stack of arranged one above another, sheets, this screw is likewise not suitable.
U.S. Pat. No. 5,597,357 discloses a self-drilling screw that has, at the end of its stem remote from the head, a drilling tip. This self-drilling screw is capable of penetrating metal sheets having a thickness of more than 2 mm.
The drawback of this self-drilling screw consists, however, in that during the setting process, chips are formed which must be removed, with substantial expenses, for corrosion reasons. In addition, a user should apply a high press-on force during the entire drilling process. Also, with formation of chips, a certain amount of material becomes lost and cannot form part of a holding force recipient.
Accordingly, an object of the present invention is to provide a thread-forming screw in which the foregoing drawbacks of the known screw are eliminated.
Another object of the present invention is to provide a thread-forming screw capable of penetrating, without formation of chips, a stack of at least two, arranged one above another metal plates with a common thickness of at least 2 mm.